An optical fibre generally comprises a silica core clad with a layer of silica or plastic cladding of lower refractive index than the refractive index of said core, so that light propagating in the core is inhibited from entering into the cladding from the core. The cladding acts as an optical isolator allowing the fibre to be fitted with a mechanical covering without loss of light from the fibre.
A mechanical covering is provided to suppress or reduce the following hazards:
Abrasion which can cause microfractures in the surface material of the optical fibre, making the fibre mechanically fragile and increasing its transmission losses;
Corrosion which can likewise cause microfractures having the same detrimental effects as abrasion;
Mechanical stress, both longitudinal and transversal, which can result in microcurving and microbreaking of the fibre during cable manufacture, lessening both the mechanical and the transmission performance of the fibre.
Mechanical coverings intended to protect optical fibres from such hazards have been made by extrusion of various plastic materials. Such extrusion processes have tended to lessen the properties of optical fibres by subjecting them to mechanical stress, thermal stress . . . A typical extrusion method surrounds an optical fibre with a plastic covering in an extruder in the general form of a tubing die, i.e. having a drawplate concentric with a punch allowing the optical fibre to pass therethrough. A screw drives previously heated and molten plastic material through the annular gap between the punch and the drawplate to form the covering surrounding the fibre as it cools and solidifies.
Two such coverings are known:
The first covering is a tight fit on the fiber. It is made by using a so-called "pressure" extruder in which the punch is wholly inside of the drawplate, allowing the extruded plastic material to come into contact with the optical fibre while still in the molten state and under high pressure. Such a process is disclosed for instance in U.S. Pat. No. 3,742,107 to Hawkins. It has the disadvantage of causing microbreakings in the fibre which seem to be due to the high lateral pressure to which it is submitted during the extrusion.
The other covering is very loose. The radial clearance between the fibre and the covering can be as much as 70 microns for a fibre of a diameter of 125 microns.
Such a covering is made with a so-called "tube" extruder in which the drawplate and the punch are extended by concentric tubular "calibrating" parts and the calibrating part of the punch extends at least about 2 mm futher than the calibrating part of the drawplate. In this arrangement the fibre is not subjected to high lateral pressure by the extruded material of the covering. However, optical fibres provided with such loose coverings suffer increased transmission losses due in particular, to microcurves facilitated by the large radial clearance and differential expansion and contraction of the fibre and the covering.
It is an object of the invention to provide optical fibres with protective coverings wherein the fibre is less subject to microbending and microbreaking, and therefore of reduced transmission losses, while still mechanically protected to the same extent.